Six patients with recent-onset, noninsulin dependent diabetes underwent assessment of glucose tolerance, insulin secretory capacity, and insulin-induced glucose disposal before and after 6-10 weeks of intensive aerobic training while maintaining body weight. Fasting plasma glucose declined in every subject (average= -33mg/dl), and oral glucose tolerance (3 h integrated plasma glucose) improved in five of the six (average=-74mg.3h/dl) after training. These improvements were associated with one or more of the following: (1) an increase in the early (30 minute) plasma insulin response to oral glucose (average=39MuU/ml), (2) an increase in the 3h integrated plasma insulin response to oral glucose (average =+33MuU.3h/ml), (3) an increase in the insulin-induced glucose disposal rate during theeuglycemic glucose clamp (average=+0.6mg/kg/min=+27%), and/or (4) a decrease in the fasting plasma insulin level (average=-11MuU/ml). Of these improvements, only the increase in early insulin response was seen in every subject. Our results show that regular endurance exercise is effective in improving glucose homeostasis and may serve as an adjunct to other modes of treatment in recent-onset noninsulin dependent diabetics.